1. Field of the Invention
The present invention relates to multilayer rigid-flex printed circuit boards for use with an infrared reflow oven and a method for assembling same. More particularly, the present invention includes an improved structure which enables the rigid-flex printed circuit boards to withstand the high temperatures of an infrared reflow oven which is used for assembling circuits in a high volume production environment.
2. Description of the Related Art.
The techniques of fabricating multilayer rigid-flex printed circuit boards are well known in the field. An early example of the prior art is disclosed in U.S. Pat. No. 3,409,732. Typically, a rigid-flex printed circuit board includes flexible sections of the printed circuit board extending from the periphery of the rigid section or sections. The rigid sections of the printed circuit boards are typically used as sites for electronic components or mechanical hardware. The printed circuit board may consist of several conductor layers which are separated by insulator layers. The conductor layer is fabricated from one continuous sheet of copper foil which is etched to form the conductor patterns or traces.
As a result of electronic packaging needs, more complex multilayer rigid-flex printed circuit boards using up to 25, or even more, conductor layers have come about. The various conductor layers are separated by layers of insulation. The various conductor layers are electrically connected by vias, or copper conductor barrels, formed in the rigid section of the circuit board through the layers of conductors and insulation. The rigid-flex printed circuit boards having many layers experienced certain problems when subjected to high temperatures. One problem resulted from the thermal expansion of the rigid section of the circuit board in the Z-direction, or thickness, resulting in fractured conductor barrels. A second problem associated with high temperatures was delamination of the rigid board section resulting from volatized absorbed moisture which was entrapped in the circuit board.
U.S. Pat. No. 4,800,461 to Dixon, et al. disclosed a multilayer rigid-flex printed circuit board which eliminated materials from the rigid section which caused the undesirable expansion in the Z-direction, and which absorbed excessive amounts of moisture. The complete disclosure of U.S. Pat. No. 4,800,461 is hereby incorporated by reference and made a part of this application. In Dixon, et al., the rigid section of the printed circuit board does not contain the troublesome materials, those being acrylic adhesive and KAPTON, because the flexible sections which include Kapton and acrylic adhesive only extend to, but not substantially into, the rigid section. Of course, the Kapton and acrylic layers can extend an insubstantial distance into the rigid section without causing undue problems. Subjecting the circuit boards of Dixon, et al. to hot solder reflow does not cause differential expansion and contraction leading to cracking of the copper plated through barrels or delamination of the layers. Additionally, extended baking times at temperatures on the order of 250.degree. F., often as long as 48 hours, to remove moisture from the Kapton and acrylic layers are avoided.
Although U.S. Pat. No. 4,800,461 solved the temperature-related problems associated with the rigid section of the board, the solution did not consider the adverse effects of high temperatures on the flexible section of the rigid-flex printed circuit board. The rigid-flex printed circuit board of U.S. Pat. No. 4,800,461 was utilized in the assembly of circuits in a low volume production environment in which the electrical components are manually soldered and only isolated areas of the rigid board were subjected to concentrated heating and thus elevated temperatures. Additionally, this circuit board was used with certain high volume production assembly processes such as wave soldering and vapor phase soldering which also did not result in sustained, elevated temperatures in the flexible materials.
In a high volume production environment utilizing infrared reflow ovens, entire circuit boards are passed through the oven which elevates the temperature across the entire board, both the rigid and flexible sections of the board. A major problem that arises when a rigid-flex printed circuit board is soldered in an infrared reflow oven is that the flexible portion of the board cannot withstand the elevated temperatures of the heat panels in the oven. As a result, the flexible section of the board will degrade by delaminating. The infrared reflow ovens, commonly referred to as IR ovens, heat by natural convection and infrared energy. The heating panels in the IR oven may reach temperatures of 430.degree. C. The flexible section of the printed circuit board begins to degrade at approximately 275.degree. C. The degradation of the flexible portion of the board will adversely affect the integrity of the entire printed circuit board Consequently, there is a need for a rigid-flex printed circuit board which can be used in high-volume production environments utilizing infrared reflow ovens without damaging the integrity of the flexible portion of the circuit board.